Effect of Process Atmosphere Dew Point and Tin Addition on Oxide Morphology and Growth for a Medium-Mn Third Generation Advanced Steel During Intercritical Annealing

The combined effects of process atmosphere oxygen partial pressure, annealing time, and a 0.05 wt pct Sn addition on the selective oxidation of a model 0.1C-6Mn-2Si third generation advanced high-strength steel (3G-AHSS) composition were investigated. External and internal oxidation of both steels were observed after intercritical annealing at 963 K (690 °C) for holding times of 60 to 600 seconds under all process atmosphere dew points explored—i.e., 223 K, 243 K, and 278 K (− 50 °C, − 30 °C, and + 5 °C). The external MnO morphology was changed from compact and continuous film-like nodules to a fine and discrete globular morphology, with thinner external oxides, for the Sn-added steel. Cross-sectional TEM analysis revealed that the Sn addition also resulted in significant refinement of the internal oxide network. Kinetic studies showed that both the external and internal oxidation followed a parabolic rate law, where the Sn addition to the steel chemistry resulted in lower external and internal oxidation rates. 3D atom probe tomography of the external oxide/steel interface showed that Sn was segregated to the interface with enrichment levels ten times the bulk value, which was concluded to be responsible for the observed morphological changes. The resultant refined external oxide structure is expected to have significant benefits with respect to reactive wetting by the continuous galvanizing bath.